U.S. patent number 7,701,164 [Application Number 10/588,281] was granted by the patent office on 2010-04-20 for control of electrical machines.
This patent grant is currently assigned to Dyson Technology Limited. Invention is credited to Andrew Charlton Clothier, Hanping Dai, Stephen Greetham.
United States Patent |
7,701,164 |
Clothier , et al. |
April 20, 2010 |
Control of electrical machines
Abstract
An electrical machine, such as a switched reluctance motor, has
a rotor and a controller arranged to energize at least one
electrically energizable phase winding in dependence on the angular
position of the rotor. The controller may employ a control law
table derived by applying a predetermined DC link voltage to the
windings. Differences between an applied DC link voltage and the
predetermined DC link voltage may be compensated by applying a
predetermined correction to the angular position of energization of
the phase winding in dependence on the value applied DC link
voltage. Such a compensation factor may be derived from a
relationship held in memory.
Inventors: |
Clothier; Andrew Charlton
(Hullavington, GB), Greetham; Stephen (Charfield,
GB), Dai; Hanping (Chippenham, GB) |
Assignee: |
Dyson Technology Limited
(Malmesbury, GB)
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Family
ID: |
31985695 |
Appl.
No.: |
10/588,281 |
Filed: |
January 27, 2005 |
PCT
Filed: |
January 27, 2005 |
PCT No.: |
PCT/GB2005/000296 |
371(c)(1),(2),(4) Date: |
March 14, 2007 |
PCT
Pub. No.: |
WO2005/076459 |
PCT
Pub. Date: |
August 18, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070278983 A1 |
Dec 6, 2007 |
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Foreign Application Priority Data
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Feb 5, 2004 [GB] |
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0402528.4 |
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Current U.S.
Class: |
318/701; 318/723;
318/717 |
Current CPC
Class: |
H02P
25/092 (20160201) |
Current International
Class: |
H02P
23/00 (20060101); H02P 6/00 (20060101) |
Field of
Search: |
;318/701,727,400.03,807,696,717,723 ;388/805,815 ;363/48 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0229873.5 |
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Dec 2002 |
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GB |
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2001-87189 |
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Apr 2001 |
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JP |
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2002-58279 |
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Feb 2002 |
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JP |
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2003-244981 |
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Aug 2003 |
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JP |
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2003-311077 |
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Nov 2003 |
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JP |
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Primary Examiner: Ip; Paul
Attorney, Agent or Firm: Morrison & Foerster LLP
Claims
The invention claimed is:
1. A method of compensating for differences between an applied DC
link voltage and a predetermined DC link voltage in an electrical
machine comprising a rotor, at least one phase winding, and a
controller configured to energize the phase winding in dependence
on an angular position of the rotor, wherein the controller
comprises a memory storing a voltage compensation map comprising a
plurality of correction factors, the method comprising: measuring
the applied DC link voltage; obtaining a correction factor by
addressing the voltage compensation map using the applied DC link
voltage; and applying the obtained correction factor to the angular
position of energization of the phase winding.
2. A method as claimed in claim 1, in which the applied DC link
voltage is measured periodically.
3. A method as claimed in claim 1, in which the applied DC link
voltage is measured when the machine is started.
4. A method as claimed in claim 1, further comprising measuring the
applied DC link voltage when the machine is connected to a power
supply but before the machine is switched on and applying the
obtained correction factor to the angular position of energization
of the phase winding on starting the machine.
5. A method as claimed in claim 4, further comprising deriving an
average value for the applied DC link voltage at the
measurement.
6. A method of controlling an electrical machine, including the
method of compensating for differences between the applied DC link
voltage and a predetermined DC link voltage as claimed in claim
4.
7. A method as claimed in claim 1, further comprising deriving an
average value for the applied DC link voltage at the
measurement.
8. A method as claimed in claim 7, in which the step of deriving
the average value includes applying a filter to the applied DC link
voltage.
9. A method of controlling an electrical machine, including the
method of compensating for differences between an applied DC link
voltage and a predetermined DC link voltage as claimed in claim
1.
10. A controller for an electrical machine, wherein the electrical
machine comprises a rotor and at least one phase winding and the
controller comprises a memory storing a voltage compensation map
comprising a plurality of correction factors, the controller being
configured to: energize the phase winding in dependence on an
angular position of the rotor; obtain a correction factor by
addressing the voltage compensation map using a value of an applied
DC link voltage; and apply the obtained correction factor to the
angular position of energization of the phase winding.
11. A controller as claimed in claim 10, in which the memory
comprises a predetermined advance angle map representing the
energization of the phase winding with respect to the angular
position of the rotor over a range of rotor speeds.
12. A controller as claimed in claim 11, in which the memory
further comprises an angle correction factor to be applied to a
predetermined portion of the predetermined advance angle map, which
correction factor relates to the difference between the measured
input power and a predetermined power.
13. An electrical machine incorporating a controller as claimed in
any one of claims 10, 11, and 12.
14. An electrical machine as claimed in claim 13, in the form of a
switched reluctance motor.
15. A cleaning appliance comprising the switched reluctance motor
of claim 14.
16. A cleaning appliance incorporating an electrical machine as
claimed in claim 13.
Description
FIELD OF THE INVENTION
This invention relates to controlling an electrical machine,
particularly a machine of the switched reluctance type, such as a
switched reluctance motor.
BACKGROUND OF THE INVENTION
Switched reluctance machines have become increasingly popular in
recent years. In a switched reluctance motor, a stator has sets of
poles that are sequentially energised to rotate a rotor into line
with the energised pair of poles, under the influence of the
magnetic fields associated with each set of poles. By rapidly
switching between different pairs of poles, it is possible to cause
the rotor to rotate at a very high speed.
Recent developments in switched reluctance motors have resulted in
higher speeds of rotation of the rotor than was achievable
hitherto. However, control of the rotor at such high speeds can be
problematic. In particular, the angular position of the rotor at
which the poles are energised and de-energised needs to be
controlled carefully
It has been proposed to employ control law tables, held in a memory
associated with the control circuits of the machine. The control
law tables typically comprise look-up tables relating the turn-on
and turn-off angles to the speed and torque of the machine over a
wide range of operating conditions. However, such control law
tables are normally derived by assuming a constant value for the
voltage applied to the windings, known as the DC link voltage. In
practice, changes in the voltage of the mains power supply, as well
as other electrical changes in the environment, result in a DC link
voltage that varies over time.
Various proposals have been made to compensate for variance of the
applied voltage. For example, in U.S. Pat. No. 6,586,904, it is
proposed to sample the DC link voltage, the speed of rotation of
the rotor and the torque produced by the motor. These measurements
are then used to derive a compensated speed value and a compensated
torque value. A look-up table is then employed in order to derive
desired operating parameters based on these values.
SUMMARY OF THE INVENTION
The invention provides a method of compensating for differences
between an applied DC link voltage and a predetermined DC link
voltage in an electrical machine having a rotor, at least one phase
winding and a controller arranged to energise the phase winding in
dependence on the angular position of the rotor, the method
comprising the steps of measuring the applied DC link voltage and
applying a predetermined correction to the angular position of
energisation of the phase winding in dependence on the value of the
applied DC link voltage
The invention permits control of an electrical machine in a more
straightforward manner than was achievable hitherto, whilst also
utilising less memory.
Preferably, a predetermined relationship between the applied DC
link voltage and the correction to the angular position is stored
in a memory associated with the controller.
The DC link voltage may be measured periodically and/or on starting
the machine. The DC link voltage may also be measured when the
machine is connected to a power supply, such as a mains supply, but
before the machine is switched on.
The invention further comprises a controller for an electrical
machine comprising a rotor and at least one phase winding, the
controller being arranged to energise the phase winding in
dependence on the angular position of the rotor, the controller
further being arranged to apply, on application of a DC link
voltage, a predetermined correction to the angular position of
energisation of the phase winding in dependence on the value of the
applied DC link voltage.
The controller may be incorporated in a variety of electrical
machines, for example a cleaning appliance.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described, by way of example, with
reference to the accompanying drawings, in which:--
FIG. 1 schematically shows a typical switched reluctance motor;
FIG. 2 shows a prior art control map for the motor of FIG. 1;
FIG. 3 is a schematic diagram of apparatus for generating a control
map for the motor of FIG. 1;
FIG. 4 illustrates a graph of desired input power over a range of
operating speeds of the motor of FIG. 1;
FIG. 5 illustrates a control map for controlling the motor of FIG.
1;
FIG. 6 is a schematic diagram of alternative apparatus for
generating a control map for the motor of FIG. 1;
FIG. 7 schematically shows the motor of FIG. 1 and a control
circuit;
FIG. 8 shows a typical wave form of the DC link voltage;
FIG. 9 illustrates a relationship between the applied DC link
voltage and a correction to the angular position of the rotor at
which the phase windings are energised; and
FIG. 10 shows a cleaning appliance in the form of a cyclonic vacuum
cleaner incorporating the motor of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Like reference numerals refer to like parts throughout the
specification.
FIG. 1 is a cross-sectional view of a typical switched reluctance
motor. It comprises a rotor 1, mounted on a shaft 2 and a stator 3.
The rotor 1 comprises an axially laminated stack of steel plates,
arranged to form a pair of poles 1a, 1b. The stator 3 comprises a
stack of steel laminations arranged to have, in this example, four
inwardly projecting salient poles 3a, 3b, 3c and 3d. Opposing poles
3a and 3c each support a winding 4a, 4b which together form a first
phase. The other diametrically opposite poles 3b and 3d similarly
accommodate respective windings 4c and 4d, which represent a second
phase. Each winding 4 comprises a large number of turns (e.g. 50+
turns) of an insulated electrical conductor around the respective
stator pole.
In use, energisation of the phase windings is controlled in order
to effect rotation of the rotor. Thus, it is imperative that the
rotational position of the rotor with respect to the phase windings
is known. Thus, position detecting means are provided, in this case
in the form of an encoder disk 5, source 6 of optical radiation and
an optical sensor (not shown). The encoder disk 5 is positioned
between the source 6 and detector, the plane of the disk being
substantially perpendicular to the direction of optical radiation.
Apertures in the disk allow light from the source to be transmitted
to the sensor. As the encoder disk 5 rotates with the shaft 2 of
the rotor assembly 1, light from the source is interrupted
intermittently. Thus, the optical sensor receives a pulsed light
signal. Signals from the optical sensor are transmitted to a
controller.
At low speeds, it is relatively straightforward to control the
application of voltage to the phase windings. Typically, this is
done by means of pulse width modulation (PWM), which is discussed
further below. However, as speed increases, the angular position of
the rotor at which voltage is applied to the windings (the turn-on
angle) must be advanced, as must the angular position at which the
application of voltage is stopped (the turn-off angle). The turn-on
angle must be advanced to allow the build-up of flux in the winding
from zero to the desired value before the inductance starts rising
as the poles approach. This is known as the on-advance angle.
Similarly, the turn-off angle must be advanced to be able to reduce
the flux to zero before inductance starts diminishing as the poles
separate. This is known as the off-advance angle.
In a typical controller for a switched reluctance motor a control
law map is employed in the form of a look-up table. An example of
such a table is shown in FIG. 2. The table comprises a series of
storage locations held in a memory. The table charts the
relationship between the speed of the motor and the desired torque
produced by the motor. In each location of the table are stored
control parameters for controlling the machine to produce the
corresponding speed and torque. Typically, the control parameters
comprise the on-advance angle and the off-advance angle. During
operation of the motor, the speed and motoring torque are measured
and input to the control system, which employs the look-up table to
find the appropriate firing angles to control energisation of the
phase windings in order to achieve a desired speed and torque.
However, a drawback of this type of control map is that it occupies
a large amount of memory. Furthermore, if the control map is
applied in manufacture to a batch of motors, it is essential that
those motors have the same performance characteristics in order to
achieve consistent results. Therefore, the motors must be
manufactured from components having consistent and defined
tolerances, both physical and electrical. Naturally, this adds
considerably to the overall cost of the motor. The alternative is
to generate look-up tables from scratch for each motor, which
proposal is extremely time-consuming and also costly.
Generating a Control Map
A control map overcoming this problem, and a method of generating
the control map will now be described with reference to FIGS. 3 to
5.
A schematic diagram of apparatus suitable for generating a control
map according to the invention is shown in FIG. 3. The motor is
indicated generally by the reference numeral 7 and is located in a
motor bucket 8, together with an electrical control board 9. The
arrangement includes a voltage source 10 in the form of a DC power
supply that can be either a battery or rectified and filtered AC
mains. The DC voltage provided by the power supply 10 is supplied
across a DC link and switched across phase windings of the motor 7
by the electronic control board 9. In the present application, the
DC voltage provided to the switched reluctance machine (whether
from a battery, rectifier or otherwise) is referred to as the "DC
link voltage".
The control board 9 is connected to each of the phase windings of
the motor 7, and controls operation of the motor by causing the
phase windings to be energised in sequence. A power meter 11 is
connected to the DC link to measure the input power. Signals from
the power meter 11 are input to a test controller 12 which, in
turn, sends data to the electronic control board 9.
FIG. 4 shows a desired input power profile over a wide range of
operating speeds. Such a profile may predetermined by means of
modelling software, and may be generated for a specific application
of the motor. For example, the profile of FIG. 4 has been generated
for a vacuum cleaner motor. This profile shows the power increasing
steadily with speed until maximum power is achieved at very high
speeds of between eighty thousand and one hundred thousand rpm. At
speeds above this limit, the motor is arranged to power-down to
avoid excessive wear to the components.
FIG. 5 shows a corresponding predetermined profile in the form of a
nominal advance angle map. This profile represents an idealised
operating condition. The line indicated by the reference numeral 13
shows the variance of the off-advance angle with increasing speed
of the rotor. The line indicated by the reference numeral 14 shows
the relationship between the on-advance angle with rotor speed. The
on-advance angle does not vary at all until the motor has reached
speeds in excess of 60,000 rpm. At lower speeds, energising of the
windings is mainly controlled by PWM. The line 15 on the graph
indicates PWM control, and shows the percentage of each cycle
during which voltage is applied to the phase windings. At slow
speeds of, for example, five thousand rpm, a voltage pulse is
applied for only approximately 10% of the duty cycle. The voltage
pulse widths increase as the speed increases until at around fifty
thousand rpm, full width voltage pulses are applied to the
windings.
In order to generate the control map, the voltage supply 10 is
arranged to supply a constant voltage to the motor 7 via the
electronic control board 9. The value of the applied constant
voltage is selected to correspond with a typical operating voltage
that would be supplied to the machine via the DC link in use. In
the present example of a motor for a vacuum cleaner, the constant
voltage is selected to represent the voltage of a typical domestic
mains supply, for example, 230V.
A speed is selected that corresponds to a predetermined input power
from the power speed profile. A convenient speed in this example
would be 80,000 rpm, because it is known that the motor should be
operating at full power at that speed. The control board 9 is
arranged to apply voltage pulses to the windings in accordance with
the nominal advance angle profile of FIG. 5, in order to bring the
rotor up to speed. The pulses are applied according to the
on-advance angle and off-advance angle stored in the nominal
advance angle profile.
The power meter 11 measures the input power and sends this as a
signal to the test controller 12. The controller 12 compares the
measured input power with the desired input power indicated by the
power profile of FIG. 4. If there is a discrepancy, the test
controller applies an incremental change in both the on-advance
angle and the off-advance angle, and the input power is measured
again. Again, if there is a discrepancy between the measured input
power and the desired input power, the on- and off-advance angles
are altered by another increment. A typical incremental angular
change is of the order of 0.7.degree.. This process is continued
until the measured input power and the desired input power are
substantially the same. When this is achieved, the total change in
advance angle is stored in the memory as a correction factor. In
use, this correction factor is employed over a predetermined
portion of the nominal advance angle profile, preferably the
portion during which the on-advance angle comes into play. The
advance angle profiles with a typical correction factor taken into
account are represented by the broken lines in FIG. 5. The
off-advance angle with the correction factor added to it is
indicated by the line 16. The on-advance angle with the correction
factor added to it is indicated by the line 17
Alternatively, the advance angles may be amended incrementally
until the measured input power is within a range of values with
respect to the predetermined input power.
The nominal advance angle profile and the correction factor are
held permanently in locations in a non-volatile memory associated
with the control board. The amount of data stored is typically the
equivalent of one row of data in the prior art control map
consisting of look-up tables. Thus, the control map allows a
smaller memory to be used, thereby reducing the cost of the
machine. Alternatively, the extra, unused memory may be utilised
for other applications.
A further alternative is illustrated schematically in FIG. 6. In
this arrangement, the test controller 18 is arranged to communicate
with the control board 19 by means of a radio frequency (rf)
transmitter. In this embodiment, the correction factor is
transmitted to the memory of the control board 19 by means of rf
signals. This arrangement advantageously removes the need for a
physical electrical connection between the test controller and the
control board, which is hidden within the motor bucket 8.
Voltage Compensation
The aforedescribed control map, in common with prior art control
maps assumes that the voltage applied to the windings is constant.
However, in use, the DC link voltage varies from the voltage at
which the control map was derived.
FIGS. 7 to 9 inclusive illustrate a method of compensating for a
varying DC link voltage.
FIG. 7 is a simplified schematic diagram showing a switched
reluctance motor 25 in use. In this drawing, power conversion
apparatus is indicated generally by the reference numeral 20. In
use, the power conversion apparatus 20 is connected to the mains
power supply and is arranged to provide a filtered and rectified DC
link voltage. Suitable power conversion apparatus is described in
our co-pending patent application GB0229873.5.
An example of an actual DC link voltage is shown by the line 21 in
FIG. 8. The voltage signal fluctuates rapidly with time. The DC
link voltage is sampled at the filter circuit 22 of FIG. 7, in
order to provide a smoothed average value of the DC link voltage. A
typical average DC link voltage is indicated at 23 in FIG. 8. This
average DC link value is supplied to the controller 24, in which is
stored a voltage compensation map, such as that shown in FIG.
9.
This map charts the relationship between advance angle and the
average DC link voltage. The map may be derived by experiment or
otherwise generated by means of modelling software. The map is held
permanently in a non-volatile memory associated with the controller
24. In this example, the advance angle is zero at 230V. This is
because the control map was derived whilst applying constant
voltage pulses to the windings of 230V. Thus, the control map gives
accurate control of the motor at that voltage. In this example, the
advance angle is arranged to reduce as the DC link voltage
increases, and vice versa.
When the DC link voltage is sampled from the filter, the controller
24 addresses the voltage relationship held in the memory, in order
to derive a correction factor to be applied to the advance angles
at which the phases are fired. For example, if the measured DC link
voltage is 207V, then the controller applies an advance angle
correction of to both the on and off advance angles of 2.1.degree..
Thus, the firing of the phases is controlled in a simple manner and
reduces the need for sensors for measuring characteristics of the
motor e.g. torque, speed.
The relationship between voltage and angle correction need not be
stored in the form of the map of FIG. 9. If, for example, the
relationship is a linear one, it would be within the capabilities
of the skilled person to cause the controller to apply a
predetermined correction factor to the advance angle for every
volt, or fraction of a volt, by which the applied DC link voltage
is shifted.
The aforedescribed method of voltage compensation may be applied
continuously, periodically, or simply at a predetermined event,
such as on starting the motor.
It has been found that, when the motor is connected to the power
supply, but is not being run, the DC link voltage is generally
higher than would otherwise be expected, because current is not
being drawn from the circuit. Thus, a correction factor may be
stored purely to correct firing angles at start-up of the motor.
This may be effected simply by shifting the angle compensation
factor by a predetermined voltage value. For example, on start-up,
a DC link voltage of 315V may have a corresponding advance angle
adjustment of 1.4.degree..
A further enhancement is the application of hysteresis control. If
the measured DC link voltage fluctuates rapidly between two values,
the advance angle correction factor will tend to fluctuate
accordingly. The controller may be arranged such that the change in
voltage has to be greater than a predetermined increment before the
controller applies a new value for the advance angle correction, so
that the change of advance angle lags behind the change in voltage.
For example, if the DC link voltage rises from 230V to 232V, the
controller may be configured to wait until the voltage has risen to
234V before applying a correction to the advance angle.
The invention is applicable to switched reluctance machines, and is
particularly useful in such machines that operate at high speeds
of, say, 100,000 revolutions per minute.
FIG. 10 shows one example of a vacuum cleaner 30 in which the
switched reluctance motor may be used. The motor is arranged to
drive an impeller at very high speed. The pumping action of the
impeller draws dirty air into the cleaner via a nozzle 31 and a
hose and wand assembly 32. The dirty air enters a separator 33,
which serves to separate dirt and dust from the dirty air. The
separator 33 can be a cyclonic separator, as shown here, or some
other separator, such as a dust bag. Cleaned air leaves the
separator 33 before entering the motor housing located within the
main body 34 of the cleaner. A pre-motor filter is typically placed
in the airflow path before the impeller to filter any fine dust
particles that were not separated by separator 33. A post-motor
filter may be placed in the airflow path. However, the provision of
a brushless motor reduces the requirement for such a filter. The
cleaned air is then exhausted from the cleaner to the atmosphere
via a suitable outlet.
Variations to the described embodiments will be apparent to a
skilled person and are intended to fall within the scope of the
invention. For example, while a four-pole stator, two-pole rotor
machine has been described, the invention can be equally applied to
machines having other numbers of poles on its stator and rotor and
with motors having other dimensions.
The invention is equally applicable to motors and generators, not
necessarily of the switched reluctance type, and may be employed in
appliances other than domestic vacuum cleaners, such as lawn
mowers, air conditioners, hand dryers and water pumps.
* * * * *